Quantum entanglement is a quantum mechanical phenomenon in which the quantum properties of two or more objects are described with reference to each other, even though the individual objects may be spatially separated. Two photons, A and B, are said to be entangled and form a bi-photon system when, if one property of the photon A is known, then the corresponding property of photon B is also known. For example, if two photons are prepared in a polarization entangled state, and one of the photons is observed to have a specific polarization, then the other photon will have a known polarization.
Systems for generating and detecting entangled photons have been proposed. For example, a Ti-Saphhire laser generates a coherent beam, non-linear crystals downconvert the coherent beam into frequency-entangled photons, and non-linear crystals then upconvert the entangled photons to a frequency suitable for detection. Those photons having a first state are sent along a first path, and those photons having a second state are sent along a second path. A bucket detector detects those photons sent along the first path, and a second detector detects those photons sent along the second path. Coincidence-measurements are performed on the photons detected by the first and second detectors. The bucket detector is operated as a time gate to consider the coincidences.